Digital subscriber line (DSL) technology was initially deployed to provide data-only service as a replacement for slower-speed, dial-up modems. Incumbent local exchange carriers (ILECs), competitive local exchange carriers (CLECs), and other telecommunication providers have begun to explore offering voice over DSL (VoDSL) service to deliver integrated voice and data services.
Pairgain and Added Main Line (AML) are the names of common systems used by the telephone industry to digitally add more subscriber lines to a single twisted copper pair that runs from the central office to the customer premises. These systems take digitized pulse-code modulated (PCM) data and transmit it across the twisted pair. The transmission systems use echo cancellation, time-division multiplexing (TDM), or frequency-division multiplexing (FDM) in order to transfer data in both directions using a single pair of wires.
Existing VoDSL networks may not provide the degree of reliability necessary to enable true integration of telephony and data services. A typical VoDSL network requires two basic components: (1) a gateway that links the traditional telecommunication network to the DSL network and (2) an integrated access device (IAD), residing at a customer premises, that multiplexes and processes voice and data traffic between the gateway and multiple subscriber lines. If the IAD loses power or if the gateway, IAD, or other network equipment fails, the VoDSL service is terminated. Because of these architectural limitations, existing VoDSL networks do not provide the reliability of traditional telephone systems, and as a result, telecommunication providers remain wary of adopting VoDSL service.